1. Field of the Invention
The present invention relates generally to an improved method and apparatus for the recovery of highly viscous oil in subterranean deposits. In one aspect, the invention concerns a method of resistively heating the subterranean formation to lower the viscosity of the oil. In another aspect, the invention concerns a heating and production apparatus comprising a flexible production tubing. In another aspect, the invention concerns a method of completing a well by inserting into the fluid-filled well bore production tubing modified with a buoyant body.
2. Discussion of the Prior Art
Heavy oil is naturally formed oil with very high viscosity that often contains impurities such as sulfur. While conventional light oil has viscosities ranging from about 0.5 centipoise (cP) to about 100 cP, heavy oil has viscosities that range from 100 cP to over 1,000,000 cP. Heavy oil reserves are estimated to equal about 15% of the total remaining oil resources in the world. In the United States alone, heavy oil resources are estimated at about 30.5 billion barrels, and heavy oil production accounts for a substantial portion of domestic oil production. For example, in California alone, heavy oil production accounts for over 60% of the state's total oil production. With new reserves of conventional light oil becoming more difficult to find, improved methods of heavy oil extraction have become more important. Unfortunately, heavy oil is typically expensive to extract, and conventional methods have only about 10 to 30% recovery rates of heavy oil from existing reserves. Therefore, there is a compelling need to develop a more efficient and effective means for the extraction of heavy oil.
One of the ways in which heavy oil can be recovered is through electromagnetic stimulation. This involves lowering the viscosity of heavy oil by heating it with electricity. There are several different methods of electromagnetic stimulation, including, for example, inductive heating, microwave heating, and resistive heating. Inductive heating utilizes a down-hole heating element that directly turns the current into heat. Microwave heating utilizes very high frequency energy to heat the reservoir. Resistive heating utilizes an electrode that is grounded to an adjacent well bore or to the surface. The electric current from the electrode in this method is conducted by connate brine in the reservoir. Resistive heating essentially heats the subterranean formation surrounding the heavy oil, resulting in the oil being heated and lowering its viscosity.
Electromagnetic stimulation is, in theory, the ideal way to lower the viscosity of heavy oil because of the wide availability of electricity and because it requires a minimal surface presence. However, the results have not lived up to theory. There have been many different designs for electromagnetic stimulation of heavy oil reserves, but none have worked well enough to gain widespread acceptance. This is primarily because the prior art has not developed an economical and robust downhole deployment system for electromagnetic stimulation.
Among the methods of electromagnetic stimulation, resistive heating seems to hold the most promise as a reliable means of lowering the viscosity of heavy oil. One reason for this is that resistive heating does not require any type of injection, because the current simply flows through the conductive brine of the oil well. However, as in other types of electromagnetic stimulation, there has yet to be a widely accepted system for resistive heating. Thus, there remains the need for an electromagnetic heating system that is effective in increasing the productive output of heavy oil reservoirs.
Oil and/or natural gas wells are often drilled horizontally in several directions from one well head for a variety of reasons. However, one problem with the completion of horizontal wells is that it is difficult to extend production tubing to the end of the well. Therefore, there is also a need for a method to more effectively complete a horizontal well.